1. Field of the Invention
The present invention relates to a process of manufacturing an extruder screw of the type used in a machine having a screw feed mechanism, such as in injection molding macines or extrusion machines.
2. Description of the Prior Art
At the present time there is great demand for an injection molding machine or extrusion machine which uses a screw feed mechanism. However, in recent years the types of resins to be treated have been increasing in large number and some of these resins produce corrosive gases such as fluorine gas associated with melting of the resins at the time of processing. Also, there is an increasing number of resins which contain abrasive solids, for example, resins incorporating glass fibers, carbon fibers or magnetic powders. Furthermore, these injection molding machines and extrusion machines are now being used in the processing of ceramic materials.
For the above reasons, the requirements of corrosion resistance and wear resistance for the material used as parts of the injection molding machine or the extrusion machine are becoming increasingly more severe. Of these parts, the extruder screw constitutes an important part of the screw feed mechanism since it not only transports raw resins but also performs melting and mixing thereof. Thus, the material used for the extruder screw requires a combination of sufficient strength, corrosion resistance and wear resistance. Materials which have been used for such an extruder screw include steels such as maraging steels or cold-worked tool steels, e.g. JIS type SKD-11.
The maraging steels exhibit a high strength, but do not necessarily have sufficient corrosion and wear resistance. For instance, if the processed resins include abrasive solids, such as glass fibers, the threaded part of the screw (the so-called flight) will wear faster and thus the life of the extruder screw will be shortened. Likewise, although the cold-worked tool steels may include tool steels with fine chromium carbides precipitated and disbursed in the material, these steels do not always have sufficient wear resistance against resins containing glass fibers or magnetic powders. In addition, the processing of ceramic materials cannot be achieved without even faster wear rates.
Various proposals have been suggested to eliminate the disadvantages of extruder screws made of steel, such as providing a spray coating of cobalt-based alloys or nickel-based alloys containing particles of high hardness, such as tungsten carbides, or providing a composite extruder screw and the like having a sintered alloy of these types of coatings affixed thereto (for example, such as the one disclosed in Japanese Patent Laid-Open NO. 183430/86). However, alloys containing particles of tungsten carbide have a disadvantage in that they tend to wear parts contacting these alloys (in the present instance, the internal wall of the cylinder), even if these parts are made of alloys which are wear resistant. In addition, since tungsten is a scarce resource and tungsten mines are not uniformly distributed throughout the world, such materials are more expensive and more difficult to obtain.